Polycarbonate resin materials and methyl methacrylate resin materials are currently widely used in various optical information media as optical materials of the light-transmitting layers and the like because of their moldability, transparency, and prices. One drawback of these resin materials is their lack of sufficient scratch/abrasion resistance and sufficient anti-staining property against organic stains. Another drawback is that these resin materials are easily charged because of their high insulation, so that the surfaces of the optical information media may pick up substantial amounts of dust particles during storage or use of the media, causing errors in recording/reproducing of optical information.
To improve the scratch resistance of the surface of media, a transparent, scratch-resistant hard coat is generally formed on the surface of the light-transmitting layer of the media. This is done by applying an active energy ray-polymerizable/curable compound onto the surface of the light-transmitting layer and subsequently irradiating active energy rays, such as ultraviolet rays, onto the surface to cure the compound. The active energy ray-curable compound typically includes, within its molecule, two or more polymerizable functional groups, such as (meth)acryloyl groups, that take part in polymerization. Although the hard coat obtained in this manner has higher abrasion resistance as compared to the surface of the resin films made of a polycarbonate, methyl(meth)acrylate and the like, the highest achievable abrasion resistance is still limited and the hard coat does not necessarily provide sufficient scratch resistance required during the use of the media. Use of harder resins to improve the scratch resistance generally results in an increase in the shrinkage of the hard coat upon curing, so that the resulting medium tends to suffer significant warp in the disk surface. Since the sole purpose of such hard coats is to improve the scratch resistance, the coatings generally fail to achieve sufficient anti-staining property against various contaminants, including dust particles, oil mist in the atmosphere, and fingerprints.
A hard coat having anti-staining property against organic stains is described in Japanese Patent Laid-Open Publication No. Hei 10-110118(1998). Such a hard coat can be fabricated by admixing a non-crosslinking fluorine-based surfactant to a hard coat agent. However, the non-crosslinking fluorine-based surfactant in the hard coat agent is gradually lost as the media are repeatedly cleaned by, for example, wiping over the course of their use.
It is suggested in Japanese Patent Laid-Open Publication No. Hei 11-213444(1999) to apply a fluorine-based polymer onto the surface of substrates of optical disks made of conventional materials such as polycarbonate. However, the fluorine polymer is physically adsorbed to the surface of the substrates only through the effect of van der Waals force, so that the adhesion of the fluorine-based polymer to the substrates surface is considerably weak. Thus, the surface treatment with the fluorine-based polymer coating poses a significant problem in terms of durability.
It is suggested in European Patent Publication No. EP1146510A2 to add metal chalcogenide fine particles such as silica fine particles to a hard coat to improve the scratch resistance of the hard coat. A film of a water repellant group or an oil repellent group-containing silane coupling agent is then applied over the hard coat to improve the anti-staining property of the surface.